Using conventional such as surface chemical treatment, sandblasting method (see JP-B-49-8711), coating with a light-diffusing white coating material (see, for example, JP-B-34-9168), etc., it is difficult to obtain on the surface of a plastic material a thin light-diffusing layer which can be formed partly or in the shape of a pattern, permits adjustment of light diffuse reflection properties and light diffuse transmission properties, and can be formed not only on planar surfaces but also on any curved surfaces. The reason for the difficulty is explained below for each method.
In the surface chemical treatment, a violent reaction is carried out using a strong acid, a strong base, etc., so that the spectral absorption characteristics of the surface of the resulting light-diffusing layer are changed by the chemical reaction. In other words, coloring and the like are caused. Moreover, the chemical treatment merely forms depressions and protuberances in the surface and hence does not impart significant light diffuse reflection properties or light diffuse transmission properties to the surface. Furthermore, when the treatment is carried out so as to form a pattern, it is difficult to prepare a resist which is resistant to the above-mentioned violent chemical reaction.
In the sandblasting method, sand grains are caused to collide with a plastic material at high speeds. For the collision of the grains, high energy is required. In addition, the finer the sand grains, the greater the air resistance, so that the fineness of the plastic material surface achieved by the treatment has its own limit and it cannot be expected to impart very high light diffuse reflection properties or light diffuse transmission properties to the surface of the plastic material.
When a light-diffusing white coating material is applied on a plastic material, the spectral absorption characteristics of the coated material tend to be imbalanced because the light-diffusing white coating material is composed of a white pigment, a binder, various additives, etc. The reason is that in the case of such a coating material, coating properties should be given priority. It is difficult to make the content of the white pigment in the coating material much higher than that of the binder from the viewpoint of the film-forming properties of the coating material. In addition, in this case, it is the boundary surface between the binder and the white pigment that scatters light. A small difference between their refractive indexes results in a small light-scattering effect. Therefore, if a large light diffuse reflection effect is desired, it is necessary to thicken the coating of the light-diffusing white coating material, so that a thin light diffuse reflective layer cannot be realized. Moreover, the light-diffusing white coating material is difficult to be applied on a curved surface.
On the other hand, conventional thin light boxes requiring a uniform illumination surface includes the following two types of light boxes: the so-called edge light type (see, for example, JP-A-57-128383 and JP-A-2-126501) which comprises a light-source provided at a side end of an illumination surface, and a light-quantity-controlling member with a light guide panel having a reflective pattern formed thereon, which member diffuses light from the light source uniformly on the illumination surface; and the so-called lighting curtain type (see, for example, JP-B-59-8809) which comprises a flat housing with an illumination surface on its open side, a light source provided in the flat housing and a light-quantity-controlling member provided right above the light source and containing a translucent reflector called "lighting curtain" which has a reflecting pattern, so as to allow light from the light source to reflect repeatedly between the inner surface of the housing and the light-quantity controlling member to achieve uniform illumination.
The edge light type light box is disadvantageously heavy because it is used together with a light guide panel made of transparent plastic, glass or the like. The edge light type light box is disadvantageous in that the area of light guide portion cannot be increased in proportion to the enlargement of the illumination surface since light is introduced through the end face of the light guide panel, so that the light box is dark.
The lighting curtain type light box is advantageous in that the light box is lightweight and very light because no light guide panel is used therein. However, its assembling is complicated because the positional relationship between the illumination surface and the reflective surface at the inner surface of the housing must be strictly set to obtain a uniform illumination.
In both the edge light type light box and the lighting curtain type light box, a regularly reflecting pattern such as a conventional aluminum-deposited pattern as well as a printed pattern with a white pigment is conventionally used as the reflecting pattern on the light-quantity-controlling member. However, the regularly reflecting pattern has the shortcoming that parallax is caused on the illumination surface, depending on the angle from which the illumination surface is looked at, due to its directivity, although the regularly reflecting pattern has a high reflectance. The printed pattern with a white pigment is disadvantageous in that the reflection wavelength characteristics of the pigment are shifted by various additives added to an ink for printing in order to maintain the printability, resulting in the so-called color shift. Furthermore, in the case of a white pattern obtained by printing, the size of dots constituting the pattern is restricted by printing techniques and cannot be made very small. Therefore, the illumination surface requires an additional light diffuser panel for hiding a dot image since printing of dots with an invisible size is difficult.